The U.S. intelligence community is vast, composed of 17 distinct organizations each operating under its own shroud of secrecy.
Oversight of these agencies generally falls to the Department of Defense or Congress, leaving the average citizen with precious little knowledge of how they operate.
Before ex-NSA contractor Edward Snowden's leaks, we knew very little about their budget, mission, or capabilities. 
This is what we know so far about the secretive U.S. intelligence community.